Lock-in amplifiers are a form of detector particularly well suited to extract low strength narrowband signals from broadband noise. Because the lock-in amplifiers are phase sensitive, they are able to extract an input signal component at a specific frequency and phase by multiplying a reference signal against the broadband input signal. The reference signal may be from an oscillator or function generator, at any appropriate waveform (e.g., a sine wave, a square wave etc.). FIG. 1 illustrates a typical single reference lock-in amplifier 110 that is known in the prior art, where the reference from VCO (variable crystal oscillator) 115 is mixed with the input signal in mixer 120. The resulting signal may also be narrowband filtered, in the illustrated case by use of a low pass filter (LPF) 125 following mixer 120.
It is sometimes desirable to use an external reference in a lock-in amplifier. In the typical case, the VCO of the lock-in amplifier is phase-locked to the external reference, and FIG. 2 illustrates one approach to locking the internal reference signal to an external reference. As shown in FIG. 2, system 205 includes a reference 206, which provides an external reference signal to lock-in amplifier 210. System 205 also provides an input signal to lock-in amplifier 210, which includes mixer 220, LPF 225 and PLL 215 (which is further depicted in FIG. 2 to include mixer 212, integrator 213, and VCO 211). While two references are used, one external (e.g., reference 206) and one internal (e.g., PLL 215), there is still only one lock-in signal applied to the mixer 220 to extract the signal of interest from the input signal.
In addition to single lock-in amplifiers, some have suggested the use of a double lock-in to minimize noise issues at the frequency of interest. One such approach is illustrated in the article by J. Goree, “Double lock-in detection for recovering weak coherent radio frequency signals,” Rev.Sci.Instrum., Vol. 56, No. 8 (August 1985). In that case, it was found that significant and problematic RF pick-up was passing unattenuated through the lock-in, rendering lock-in detection useless. By introducing a second lock-in device before the first one, it was disclosed that the second one be synchronized to a system modulation (e.g., a mechanical chopper wheel), thereby minimizing the unmodulated RF pick-up contribution at the frequency of interest.
However, the above approaches still have a common limitation in their use of a single reference signal for the lock-in with the signal of interest. In some instances a more complex, i.e., a composite, reference signal is desired to extract a signal or signals of interest. This would allow one to extract multiple signals of interest, or avoid the particular single frequencies and harmonics of each individual reference (e.g., by composite we mean a combination of two reference signals thereby yielding inter-modulation and/or its sideband/harmonic components). The single reference lock-in approach of the prior art is unable to provide an appropriate signal for lock-in in these cases.
Thus, there is a need for an improved lock-in detector or amplifier, one which allows for flexibility and ease in achieving a lock-in via a composite reference.